Employing control units, such as shooting pots, to introduce thermoplastic resins or other materials into a mold cavity in an injection molding machine is well known. Generally, a primary resin source feeds the material to a shooting pot reservoir which is, in turn, operated to feed a measured, or metered, quantity of the material into the mold cavity. U.S. Pat. No. 3,516,123, entitled "Injection Molding Machine", to Lang; and U.S. Pat. No. 3,231,656, entitled "Apparatus and Method of Plastic Molding", to Ninneman both disclose the use of shooting pots to provide accurately metered shots of resin to a mold cavity. Metering permits an accurate amount of material to be injected into a mold to ensure that a properly formed part is created and to prevent waste of material in the form of "flash", etc. due to overfilled molds. Metering is generally achieved by controlling the distance by which an injection plunger in the shooting pot is retracted and advanced for each shot.
Other metering techniques are also well known. For example, U.S. Pat. No. 4,966,545, entitled "Staged Shooting Pot for Injection Molding, to Brown, shows how a single shooting pot can be operated to cause two sequential metered injections of the same resin into the same mold cavity. U.S. Pat. No. 4,460,324, to Van Appledom, entitled "Shot Cylinder Controller for Die Casting Machines and the Like", shows how the injection speed of the piston of shooting pot can be controlled, thereby controlling the rate of injection of the resin into the mold cavity.
It is also well known to supply thermoplastic material to a multicavity mold through a hot runner system. The hot runner system can include a plurality of shooting pots, with at least one shooting pot associated with each mold cavity.
Hot runners systems can also be used for multimaterial injection, or coinjection, molding. Typically, two or more resins are injected, either simultaneously or sequentially, into each mold cavity to produce multi-layered molded structures. For example, a common application for multimaterial molding is the production of food quality containers from recycled plastic. Government standards require that any surfaces which contact the food be made of new, virgin, plastic. To take advantage of lower cost recycled plastics, manufacturers use confection techniques to encapsulate recycled material in a sheath of new plastic. U.S. Pat. No. 5,098,274 to Krishnakumar, entitled "Apparatus for Injection Molding of Multilayer Preforms", and U.S. Pat. No. 4,717,324 to Schad, entitled "Coinjection of Hollow Articles and Preforms" both disclose injection molding machines for multimaterial applications.
Generally, individual control of the shooting pot strokes is provided in these prior art injection molding machines. Separate hydraulic actuation cylinders for each shooting pot injection plunger are mounted inside the machine's stationary platen. These hydraulic cylinders must be individually set for stroke to control the individual metering of the resins into the mold cavities. The setting of the cylinders can be a hazardous operation, which is performed manually and requires personnel to reach into the machine amongst the heated injection nozzles, close to hot surfaces and heated injection materials. Furthermore, the molding process has to be interrupted for this adjustment, which can cause significant loss of production time, especially in larger machines having up to ninety six injection plungers.
U.S. Pat. No. 4,632,653 to Plocher, entitled "Press with a Plurality of Injection Plungers" describes a common actuator for the injection plungers in a transfer molding machine. The injection plungers are actuated by a hydraulic drive acting on a single crosspiece. However, the shooting pot actuator disclosed in Plocher has several limitations and disadvantages which make it inapplicable to metered injection molding machines. Firstly, the shooting pots in a compression molding machine do not provide metered shots. Instead, each shooting pot is filled with an approximate amount of resin, and the injection pistons are actuated by the crosspiece to compress the resin into the mold cavity. Plocher discloses pressure compensating pistons and overflow channels to relieve the mold cavities in the case of overfilling, which results in non-uniform product and flashing. Also, there is no mechanism provided for adjusting the stroke of the injection pistons since precise control of the amount of resin injected into the mold is not critical in such a transfer molding process. Second, the crosspiece actuator in Plocher is located within the mold, which increases the cost of designing and manufacturing the mold. Also, such a design is impractical in machines with high clamp forces as the volume occupied by the crosspiece reduces the strength of the mold component in which it is located, thus increasing the likelihood of deformation of mold components when clamped. Further, the mold must be completely disassembled to obtain access for maintenance, adjustment, or replacement.
A device capable of actuating multiple shooting pots for metered multimaterial injection is disclosed in commonly assigned U.S. patent Ser. No. 09/050,095. The shooting pot described therein has a number of pusher rods attached to plates driven externally of the mold. The pusher rods extend through apertures in the stationary platen, and each abut against a respective injection piston to inject material into the mold. One problem, with the device as originally conceived, can occur when an injection piston seizes. If a single piston seizes, the actuator will attempt to overcome the resistance of the seized piston, potentially damaging the piston, its cylinder, or buckling its associated pusher rod. This can result in costly downtime and repair. Piston seizing is relatively common, and can occur for a number of reasons. Typically, piston seizures can be easily remedied if no permanent damage is done to the injection molding machine.
It is, therefore, desirable to provide a failsafe device for a shooting pot actuator that limits damage due to injection piston seizure. It is further desirable to provide a failsafe device that alerts an operator of an injection molding machine to a seizure condition in the machine.